Abstract
In gated radiotherapy, the accuracy of treatment delivery is determined by the accuracy with which both the imaging and treatment beams are gated. The time delay is the time between when the markers enter or leave the specified position range for amplitude based gating, or phase for phase based gating, and when an image is obtained or linear accelerator beam starts or ceases delivery. We measured time delay for three fluoroscopy systems and three linear accelerator treatment beams, varying gating type (amplitude vs. phase), beam energy, dose rate, and period. In the system we studied, the treatment beam was turned on and off later than expected by 0.08 ± 0.03s (1 SD). Retrospectively gated fluoroscopy systems had time delays in the opposite direction, indicating the images appeared to have been acquired earlier than they had in reality. The average beam on imaging time delays were −0.04 ± 0.05s (amplitude,), −0.11 ± 0.04s (phase); while the average beam off imaging time delays were −0.18 ± 0.08s (amplitude) and −0.15 ± 0.04s (phase). In this case, the asynchronous time delays for the imaging and treatment systems add linearly. We measure and model the effect of asynchronous time delays on gated deliveries and discuss their implications.
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